'The greatest murder mystery of all time'

The conspiracy theorist jogged into the Dallas traffic, pausing in the roadway to spray-paint an orange "X" on the exact spot where President John F Kennedy was shot, 40 years ago today.

Don Miller then puffed up Dealey Plaza towards the Texas Book Repository building, darting into the road once more to spray a second "X".

Three Tennessee men in their thirties, in town for a business meeting, glanced at his handiwork from the "grassy knoll", a small lawn above the road. "Head shot, neck shot," said one, identifying each fresh painted "X" in turn. The trio resumed their calculations of bullet trajectories and lines of sight, pointing at possible sniper's nests around the plaza.

Every year about two million people visit Dealey Plaza, an otherwise unremarkable patch of municipal gardening, cut through by a trio of busy roads. They come, in theory, to remember a slain president. Some talk of being on a pilgrimage.

A few shed tears for a life cut short. But the truth is that Kennedy's life is hardly felt here at all. This may be the most visited presidential site outside Washington, but what counts here is not Kennedy's presidency, but his death - a few seconds of violence during a flying campaign visit to a conservative Texas city that did not greatly love him.

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This is what brings the day-trippers, and lures visiting conventioneers to cross town between meetings: the "whodunnit" mystery of who killed JFK; the profitable fog of rumour and misinformation that has swirled since that fatal November day.

Mr Miller was cross. "That paint isn't drying fast enough," he said, looking at the smudgy orange "X"s he had sprayed. Mr Miller moved to Dallas 11 years ago, to be near Dealey Plaza. "I consider this my laboratory", he said.

According to Mr Miller he is just one of thousands studying the assassination. "This is the greatest murder mystery of all time."

They shun such talk at the Sixth Floor Museum at Dealey Plaza, a museum preserving the old book warehouse where Lee Harvey Oswald's high-powered rifle was found near a corner window.

It took 25 years for the building to become a museum. "This did not become a place of pilgrimage by design," said Jeff West, the museum's executive director. "Dallas was never comfortable with preserving the site of the death. It only opened in 1989 because people were coming, day in day out, to see where it happened."

Mr West and his staff work hard to give Kennedy's death a context, devoting large displays to 1960s pop culture, civil rights, the Cold War, even the poisonous atmosphere of 1963 Dallas, a hotbed of Right-wing extremists.

Two thirds of the museum's 380,000 annual visitors were born after the assassination. "You've got to explain who John F Kennedy was," said Mr West. Black Americans, especially older blacks, are a notable exception, seeing JFK's death as one of a series of murderous blows to the civil rights movement, from Robert Kennedy's shooting to the killing of Martin Luther King.

"African-Americans are clearly more in line with the martyrdom aspect of it - for some African-Americans, John F Kennedy died because he was pro-civil rights," said Mr West.

For many young Americans their knowledge of President Kennedy is a patchwork of famous quotations, images of a picture-book First Lady, and the Oliver Stone film JFK.

Much of JFK was not just wrong, but "dead wrong", said Mr West. The film "wove together all the major conspiracy theories, and ended up creating a new one.

"Not only was it the military-industrial complex, it was the Mafia, and the Cubans, it was everybody".

Such re-tellings fall on fertile ground. Opinion polls consistently find 80 per cent of Americans do not believe the official verdict, that Oswald - an angry Left-wing militant and gun fanatic - acted alone.

The museum does not endorse the official version, nor any alternative theory.

Such neutrality is not always popular, Mr West conceded. "If in reality Lee Harvey Oswald was this ineffectual cipher who decided to kill the president in a fit of pique, that's not nearly enough.

"In this country, we don't like things to be arbitrary. Americans want to know who to blame."

Downstairs in the museum lobby, tourists snapped pictures of each other in front of a giant photograph of the president's limousine before the assassination, giggling as they posed between the Kennedys, as if in the car.

Rob Marsh, a 25-year-old airman, described himself as a "Kennedy assassination conspiracy buff". He said: "It's hard for me to have the emotional connection, not having been born then. For me, it's that air of mystique, that we don't know the historical truth of what happened."

Christian McQuown, a young tourist from Colorado, had just visited the nearby Conspiracy Museum, a commercial establishment devoted to presidential murders and other ghoulish mysteries.

"They've got good facts," Mr McQuown said. Such conspiracies "really make you wonder what your government is up to" he added. "That's why I like it."

For older visitors the experience was more visibly emotional. Several described their reactions in strikingly personal terms.

Ken Dischler, from Wisconsin, had slipped away from a softball coaches' convention to visit the Sixth Floor Museum. He was eight when JFK was shot, hearing the news in his classroom from a grave-faced headmaster.

"I wouldn't go to any other locations where someone was slain. But there are very few moments in your childhood you remember with such clarity," he said.